Permission prompts at runtime in Deno - Mini Project: Build & Apply

Permission Prompts at Runtime in Deno

📖 Scenario: You are building a simple Deno script that reads a file and prints its content. Deno requires explicit permission to access files. You will learn how to request permission at runtime before reading the file.

🎯 Goal: Create a Deno script that requests permission to read a file at runtime. If permission is granted, read and print the file content. If permission is denied, print a message saying permission was denied.

📋 What You'll Learn

Create a variable called filePath with the value "./example.txt".

Create a variable called readPermission that requests read permission for filePath using await Deno.permissions.request.

Use an if statement to check if readPermission.state is "granted".

Inside the if, read the file content asynchronously using await Deno.readTextFile and store it in content.

Print the content variable.

In the else block, print "Permission to read the file was denied.".

💡 Why This Matters

🌍 Real World

Many Deno scripts need explicit permission to access files or network resources. This project shows how to ask for permission at runtime, making your scripts safer and user-friendly.

💼 Career

Understanding Deno's permission system is important for developers working with secure scripts or applications that interact with the file system or network.

1

Set the file path variable

Create a variable called filePath and set it to the string "./example.txt".

Deno

# Create a variable called filePath with the value "./example.txt"
# Your code here

Hint

Use const filePath = "./example.txt" to store the file path.

2

Request read permission for the file

Create a variable called readPermission that requests read permission for filePath using await Deno.permissions.request({ name: "read", path: filePath }).

Deno

const filePath = "./example.txt"

// Request read permission for filePath
// Your code here

Hint

Use await Deno.permissions.request with { name: "read", path: filePath } to request permission.

3

Check if permission is granted

Use an if statement to check if readPermission.state is equal to "granted".

Deno

const filePath = "./example.txt"
const readPermission = await Deno.permissions.request({ name: "read", path: filePath })

// Check if readPermission.state is "granted"
// Your code here

Hint

Use if (readPermission.state === "granted") to check permission.

4

Read and print file content or show denial message

Inside the if block, read the file content asynchronously using await Deno.readTextFile(filePath) and store it in a variable called content. Then print content. In the else block, print the string "Permission to read the file was denied.".

Deno

const filePath = "./example.txt"
const readPermission = await Deno.permissions.request({ name: "read", path: filePath })

if (readPermission.state === "granted") {
    // Read file content and print it
    // Your code here
} else {
    // Print permission denied message
    // Your code here
}

Hint

Use await Deno.readTextFile(filePath) to read the file and console.log to print.

Practice

1. What does Deno.permissions.query() do in a Deno program?

easy

A. It checks if a specific permission is already granted or denied.

B. It automatically grants all permissions to the program.

C. It revokes permissions from the program at runtime.

D. It lists all permissions the program will request in the future.

Solution

1. Step 1: Understand the purpose of Deno.permissions.query()

   This method is used to check the current status of a permission, whether it is granted, denied, or prompt.

2. Step 2: Compare with other permission methods

   Unlike request() which asks for permission, query() only checks the status without changing it.

3. Final Answer:

   It checks if a specific permission is already granted or denied. -> Option A

4. Quick Check:

   Permission check = A [OK]

Hint: Query checks status without changing permissions [OK]

Common Mistakes:

• Confusing query() with request()
• Thinking query() grants permissions
• Assuming query() revokes permissions

2. Which of the following is the correct syntax to request read permission for the file data.txt in Deno?

easy

A. await Deno.permissions.request("read", "data.txt");

B. Deno.permissions.request({ permission: "read", file: "data.txt" });

C. await Deno.permissions.request({ name: "read", path: "data.txt" });

D. Deno.permissions.query({ name: "read", path: "data.txt" });

Solution

1. Step 1: Identify correct method and parameters

   The request() method takes an object with a name property for permission type and optional path for file-specific permission.

2. Step 2: Check syntax correctness

   await Deno.permissions.request({ name: "read", path: "data.txt" }); uses correct async call with an object containing name: "read" and path: "data.txt". Other options misuse parameters or use query instead of request.

3. Final Answer:

   await Deno.permissions.request({ name: "read", path: "data.txt" }); -> Option C

4. Quick Check:

   Correct request syntax = B [OK]

Hint: Use object with name and path in request() [OK]

Common Mistakes:

• Passing parameters as separate arguments
• Using query() instead of request()
• Wrong property names like permission or file

3. Consider this code snippet in Deno:

const status = await Deno.permissions.query({ name: "net" });
console.log(status.state);

If the program has not requested network permission yet, what will be printed?

medium

A. "granted"

B. "denied"

C. An error is thrown

D. "prompt"

Solution

1. Step 1: Understand default permission state

   When a permission like "net" is queried before any request, Deno returns the state "prompt" indicating user has not yet granted or denied.

2. Step 2: Confirm no error thrown on query

   Querying permission does not throw errors; it returns the current state as a string.

3. Final Answer:

   "prompt" -> Option D

4. Quick Check:

   Unrequested permission state = prompt [OK]

Hint: Unasked permissions show 'prompt' state [OK]

Common Mistakes:

• Assuming default is granted or denied
• Expecting an error on query
• Confusing query() with request() behavior

4. What is wrong with this Deno code snippet?

const permission = Deno.permissions.request({ name: "read" });
if (permission.state === "granted") {
  console.log("Access allowed");
}

medium

A. Missing await before Deno.permissions.request call.

B. Using wrong permission name "read" instead of "readFile".

C. Cannot check permission.state directly; must use a callback.

D. request() does not return a permission status object.

Solution

1. Step 1: Check async usage of request()

   The request() method returns a Promise, so it must be awaited to get the permission status object.

2. Step 2: Identify missing await impact

   Without await, permission is a Promise, so accessing permission.state is invalid and causes errors.

3. Final Answer:

   Missing await before Deno.permissions.request call. -> Option A

4. Quick Check:

   Always await request() for permission object [OK]

Hint: Always await async permission requests [OK]

Common Mistakes:

• Forgetting await on async calls
• Confusing permission names
• Trying to use callbacks instead of promises

5. You want to write a Deno script that reads a file only if the user grants permission at runtime. Which approach correctly handles permission requests and reading the file?

hard

A. const status = Deno.permissions.query({ name: "read", path: "file.txt" });
if (status.state === "granted") {
const data = Deno.readTextFileSync("file.txt");
console.log(data);
}

B. const status = await Deno.permissions.request({ name: "read", path: "file.txt" });
if (status.state === "granted") {
const data = await Deno.readTextFile("file.txt");
console.log(data);
} else {
console.log("Permission denied.");
}

C. await Deno.readTextFile("file.txt");
console.log("File read without permission check.");

D. Deno.permissions.request({ name: "read" });
const data = await Deno.readTextFile("file.txt");
console.log(data);

Solution

1. Step 1: Request permission with await and check result

   const status = await Deno.permissions.request({ name: "read", path: "file.txt" }); if (status.state === "granted") { const data = await Deno.readTextFile("file.txt"); console.log(data); } else { console.log("Permission denied."); } correctly awaits the permission request and checks if the state is "granted" before proceeding.

2. Step 2: Read file only if permission granted

   It reads the file asynchronously only after permission is granted, otherwise it informs the user permission was denied.

3. Step 3: Identify errors in other options

   const status = Deno.permissions.query({ name: "read", path: "file.txt" }); if (status.state === "granted") { const data = Deno.readTextFileSync("file.txt"); console.log(data); } uses query() which does not request permission; await Deno.readTextFile("file.txt"); console.log("File read without permission check."); reads without permission check; Deno.permissions.request({ name: "read" }); const data = await Deno.readTextFile("file.txt"); console.log(data); does not await request() and reads file regardless.

4. Final Answer:

   const status = await Deno.permissions.request({ name: "read", path: "file.txt" }); if (status.state === "granted") { const data = await Deno.readTextFile("file.txt"); console.log(data); } else { console.log("Permission denied."); } -> Option B

5. Quick Check:

   Request permission, await result, then read file [OK]

Hint: Always await request() and check state before file access [OK]

Common Mistakes:

• Using query() instead of request() to ask permission
• Not awaiting async permission requests
• Reading files without permission check